The present invention relates to nuclear cameras, also known as scintillation cameras, gamma cameras, or Anger cameras. The invention finds particular application in conjunction with multiple rotating head cameras, particularly SPECT cameras and will be described with particular reference thereto.
SPECT cameras commonly include a plurality of detector heads, e.g. three heads, which are mounted for rotation about a subject receiving examination volume. Typically, the heads can be moved closer together or further apart to accommodate different size subjects and different types of examinations.
Each of the heads includes a plate of scintillation material having a radiation receiving surface of about 30-50 cm .times.60-90 cm. The scintillation material, typically a relatively thick crystal, is mounted closely adjacent a plurality of photomultiplier tubes. Each time a radiation event occurs within a subject, it sends out radiation which strikes the scintillation crystal causing a scintillation or flash of light. The electronic circuitry connected with the full photomultiplier tubes analyzes the output of the photomultiplier tubes and provides an indication of the coordinates on the scintillation crystal face at which each scintillation occurs. From the location on the scintillation crystal at which each scintillation is detected, and the angular position of the detector head around the subject, a three-dimensional map of radiation sources in the subject is conventionally derived.
In order to coordinate the location of a scintillation event with a region of the subject, the radiation must travel to the scintillation crystal along a predictable trajectory. To this end, collimators are mounted to the detector heads to control the trajectories along which radiation must travel in order to reach the scintillation crystals. The collimators are traditionally a grid of lead vanes that are mounted on the face of the scintillation crystal. Typically, a plurality of interchangeable collimators are provided for different studies. The collimators can have vanes of different lengths, grids with different densities of vanes, vanes which define trajectories perpendicular to the scintillation crystal face, vanes which define trajectories along a cone having a common apex or origin, trajectories along cones with pieces different distances from the detector head, trajectories along divergent fans which magnify in only one direction, or the like. The collimators, which typically weight several hundred kilograms, have a wide variety of substantial weights. Moreover, the detector heads themselves contain significant amounts of lead to shield the components from stray radiation.
The large interchangeable masses of the collimator and the detector heads, the rotation of the heads, the radial adjustment of the heads, and other mechanical operations place tremendous strains on the mechanical support structures. These and other factors can cause the heads to become misaligned. It is to be appreciated, that the reconstruction of the physical positions of the radiation sources assumes that radiation is being received by the detector heads along preprescribed trajectories. Any error in the actual trajectory along which radiation is received will cause a loss of resolution and other defects in the resultant reconstructed image. Any mechanical misalignment of the detector head due to camming, tipping, errors in radial positioning or the like, cause the actual trajectory to differ from the trajectories prescribed by the reconstruction algorithm.
More specifically, small errors in the alignment of the actual and mathematical trajectories tended to cause blurring and enlargement of the apparent location of radiation sources. Greater misalignment could cause the apparent location of each radiation source to diverge into a ring of most probable locations. Further misalignment could cause still further errors, uncertainty, and loss of resolution in the resultant image, as well as artifacts which could lead to erroneous medical diagnoses.
Heretofore, checking the alignment and positioning of the detector heads was a time consuming and tedious operation. Once any mechanical misalignment was measured, manual adjustment by the technician or a repair engineer was needed to optimize the alignment.
The present invention contemplates a new and improved automatic calibration procedure which enables the reconstruction algorithm to be recalibrated automatically after conducting a single scan procedure.